In order to increase the achievable data rates for Digital Subscriber Line (DSL) services, multi-line ADSL modulation was presented in U.S. Pat. No. 6,507,608. It describes the use of the so called phantom mode for transmission or reception of data signals. A central office equipment, such as a Digital Subscriber Line Access Multiplexer (DSLAM), provides DSL services to a subscriber over a first and a second twisted pair (also called DSL bonding), as well as over a virtual (or phantom) channel defined by the difference in common-mode voltages between the first and second twisted pairs. The Customer Premises Equipment (CPE) at the subscriber's premises is coupled to the first and second twisted pair by a first and second transformer respectively. A third transformer is coupled to the center taps of the secondary windings of the first and second transformer to create said phantom channel. The first and second twisted pairs may be used for downstream communication, while upstream communications are transmitted via the phantom channel. Alternatively, all three available channels can be used in frequency division duplexing, i.e. for both upstream and downstream communication.
An alternative embodiment of phantom-mode transmission is described in US patent application 2006/268966. In this application the phantom mode is defined as being the differential signal between a twisted pair and a common ground. However, providing such phantom channel derived from a common ground is known to cause a large amount of egress noise and to be highly susceptible to ingress.
The prior art coupling arrangement for phantom-mode transmission is now described with reference to FIGS. 1 and 2 wherein:
FIG. 1 represents a typical coupling arrangement for coupling a transmitter to a transmission line, and
FIG. 2 represents a typical coupling arrangement for coupling three transmitters over two transmission lines using phantom-mode transmission.
There is seen in FIG. 1 a typical coupling arrangement for coupling a transmitter A to a transmission line L1.
The transmitter A comprises, as an illustrative embodiment, a Digital Signal Processor (DSP) DSPA for digitally encoding a data stream into data symbols to be transmitted over the line L1. The data stream is typically encoded in the frequency domain over a plurality of carriers (or tones), e.g. by means of Quadrature Amplitude Modulation (QAM). The transmit data symbols are fed towards a Digital-to-Analog Converter (DAC) DACA as digital time-domain samples. The time-domain samples are fed through a data bus comprising n lines, with n denoting the number of bits used for quantifying the individual samples.
The DAC DACA converts the digital samples into an analogue differential signal comprising two complementary and fully-balanced signals denoted as +VA and −VA. The differential signal is fed to a differential amplifier AMPA with high Common Mode Rejection Ratio (CMRR) through input terminals 1 and 2. The differential amplifier AMPA amplifies the difference between the two complementary signals +VA and −VA, namely 2 VA, and outputs two amplified complementary and fully-balanced signals +VA′ and −VA′ through output terminals 3 and 4.
The coupling unit COUP1A is a specific transformer arrangement comprising two terminals 101 and 102 coupled to the differential outputs 3 and 4 of the differential amplifier AMPA, and two terminals 103 and 104 coupled to the line L1.
The line L1 is a twisted copper pair comprising a first and second wire indistinctly referred to as the tip and ring of the line L1, and respectively denoted as L1-T and L1-R.
The coupling unit COUP1A comprises a first primary winding W1A, a first capacitor C1A and a second primary winding W2A serially coupled between the terminals 101 and 102, and a first secondary winding W3A, a second capacitor C2A and a second secondary winding W4A serially coupled between the terminals 103 and 104. The windings W1A, W2A, W3A and W4A are magnetically coupled to each other. The primary windings W1A and W2A are matched to each other, and so are the secondary windings W3A and W4A.
The coupling unit COUP1A is primarily for passing the amplified differential signal towards the line L1, and yields two complementary and fully-balanced signals +VA″ and −VA″ over the line L1.
The coupling unit COUP1A is for further isolating the transceiver circuitry from any DC component present over the line L1.
Also, any common-mode signals present on the differential amplifier output (mostly on account of component mismatches within the differential amplifier) is fully rejected by the transformer arrangement as they do not yield any current over the primary windings.
Finally, the combination of the capacitor CIA with the mutually-coupled inductors W1A/W3A and W2A/W4A and the further capacitor C2A forms a third-order high-pass filter, e.g. for filtering out any unwanted signals present in the Plain Old Telephony Service (POTS) frequency band.
There is now seen in FIG. 2 a typical coupling arrangement for coupling three transmitters A, B and C over two transmission lines L1 and L2 using phantom-mode transmission.
The transmitters A and B are respectively coupled to the lines L1 and L2 as aforementioned in FIG. 1 with alike reference numerals.
The coupling unit COUP1A, respectively COUP1B, has been slightly modified with the single capacitor C2A, respectively C2B, replaced by two serially-coupled matched capacitors with total equivalent capacitance, and with a further terminal 5, respectively 15, coupled to the inter-connection track between the two capacitors. The resulting coupling units are denoted as COUP1A′ and COUP1B′ respectively.
The third transmitter C produces two complementary and fully-balanced signal +VC and −VC to be added as common-mode signals to both the tip and ring of lines L1 and L2 respectively. As the transformer arrangement does not pass common-mode signals, the terminals 123 and 124 of the coupling unit COUP1C are connected to the terminals 5 and 15 respectively, and evenly add to both the tip and ring of the lines L1 and L2 by means of the two serial capacitors, thereby yielding +VA″+VC″ and −VA″+VC″ on line L1, and +VB″−VC″ and −VB″−VC″ on line L2.
The disclosed coupling arrangement is disadvantageous in that all those components require board space, limiting the total number of lines that can be equipped on a line termination unit, and in that the total Bill Of Material (BOM) for phantom-mode transmission is increased.
An additional drawback of the known coupling arrangement is that the power consumed by the third amplifier AMP1C is equal to that of the two other amplifiers AMP1A and AMP1B.